Method and System for Semi-Chemical Pulping

ABSTRACT

Method and apparatus for minimizing adherence of lignin to an extraction screen in a semi-chemical pulping process. The method and apparatus generally relate to supplying white liquor and NaOH to as to increase the pH of cooking liquor at or near an extraction screen.

CROSS RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/954,446 filed Aug. 7, 2007, the entirety ofwhich is incorporated by reference.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention generally relates to a method and system formaking a semi-chemical pulp from lignocellulosic material.

Semi-chemical pulping processes are known. See, e.g., U.S. Pat. No.4,229,251 to Temler; U.S. Pat. No. 4,073,678 to Hammond et al.Semi-chemical pulping processes have different requirements anddifferent operating conditions than pulping processes involvingLo-Solids® Cooking, such as those disclosed in U.S. Pat. No. 5,489,363to Marcoccia et al.

When making semi-chemical pulps (pulps with yields in the 65-85% yieldrange, typically used for production of corrugated medium used in boxmaking), several different chemicals can be used, such as NaOH, Na₂CO₃,Na₂SO₃, kraft white liquor or kraft green liquor. In some instances,high temperatures (greater than 160° C.) and short cooking times (anhour or less) may be used for making semi-chemical pulps.

In a stand alone semi-chemical pulp mill, NaOH, Na₂CO₃, and Na₂SO₃ maybe used separately or in combination or subcombinations. These chemicalsmay be advantageous because they have a minimum smell and may beregenerated in relatively simple recovery systems typically used forkraft liquor, especially when NaOH and/or Na₂CO₃ are used. Each chemicalor combination thereof may produce its own pulp characteristics from thedifferent types of wood used (e.g., various types of hardwood).Accordingly, the particular combination or subcombination may beselected based on the individual criteria of a producer.

The simple process and relatively short retention time may make itimpractical to try to perform multi-stage cooking processes.

When a semi-chemical production facility is operated in a kraft mill, itmay make sense from a chemical make-up point of view to use greenliquor. Green liquor is produced by combusting kraft black liquor anddissolving the resultant smelt into white liquor. White liquor generallyincludes a mixture of NaOH and Na₂S, and green liquor includes thosechemicals in addition to the chemicals in the smelt resulting from thecombustion of the black liquor. When using green liquor in a digesterequipped with extraction screens, through which the spent liquor isextracted, quite often the extraction screens become coated with lignin.

This clogging or coating may be caused by lignin that has precipitatedout of solution due to the lower pH at the end of the cook. The pHdecrease may result from the consumption of carbonate ion (CO₃ ²⁻). Asthe solubility of lignin, which as been exposed to alkaline cookingconditions, decreases with lower pH, it may form a sticky tar-likesubstance, which coats the digester screens and may make themnon-functional to the point that the digester has to be shut down andcleaned.

To combat this potential problem, many mills with this type of equipmentmay add substantial amounts of kraft white liquor with the green liquor(e.g., up to 20-30% of the total chemical charge), in order to keep thepH higher at the end of the cook. A higher pH at the beginning of thecook, however, may have a negative impact on the pulp yield and may makethe operation less economical, as white liquor is more expensive toproduce than green liquor.

BRIEF DESCRIPTION OF THE INVENTION

In an aspect, certain embodiments of the present invention generallyrelate to a method for inhibiting fouling of a black liquor extractionscreen in a cooking vessel for semi-chemical pulping. The method maycomprise the steps of: feeding lignocellulosic material and green liquorto the cooking vessel for semi-chemical pulping; cooking thelignocellulosic material in the cooking vessel for semi-chemical pulpingsuch that some or all lignin is separated from the lignocellulosicmaterial, and wherein lignin is dissolved into a cooking liquor;extracting via an extraction screen black liquor comprising dissolvedlignin, wherein the black liquor is formed during the semi-chemicalpulping process from the cooking vessel; and adding white liquor or abase to the cooking vessel at or near the extraction screen, such that apH at or near the extraction screen is elevated and inhibits lignin fromadhering to the extraction screen.

In an aspect, certain embodiments of the present invention generallyrelate to a cooking vessel for a process of semi-chemical pulping oflignocellulosic material that inhibits fouling of a black liquorextraction screen in the cooking vessel. The cooking vessel maycomprise: a first input conduit to the cooking vessel that carries greenliquor; a second input conduit to the cooking vessel that carrieslignocellulosic material; a cavity in which lignin is separated from thelignocellulosic material and dissolved into a cooking liquor; anextraction screen connected to an extraction conduit that extracts blackliquor comprising dissolved lignin, wherein the black liquor is formedduring the semi-chemical pulping process from the cooking vessel; and athird input conduit for delivering white liquor or a base to the cookingvessel at or near the extraction screen, such that a pH at or near theextraction screen is elevated and inhibits lignin from adhering to theextraction screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a digester in accordance with an embodiment of thepresent invention.

FIG. 2 illustrates a digester in accordance with another embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, certain embodiments of the present invention generallyrelate to increasing the pH at the end of the cook. This may keep thescreens cleaner for a longer period of time. The increased pH may beaccomplished by adding white liquor or NaOH (or any other base) througha circulation and/or central pipe at the end of the cook.

The pH of the cooking liquor (e.g., black liquor with lignin) at the endof the cook should be 10 or higher, preferable between 10 and 13, andmost preferably between 10 and 12.

In accordance with certain embodiments of the present invention, the pHat or near the extraction screen(s) may be elevated when compared withthe pH of liquor above the extraction screens. This, in turn, mayincrease the solubility of the lignin at or near the extractionscreen(s).

In certain embodiments, the pH level is raised of a liquid at or near anextraction screen of a vessel in which semi-chemical pulping isperformed. In certain embodiments, this may cause an increase in thesolubility of lignin and may also decrease the tendency of an extractionscreen to clog.

FIG. 1 shows a portion of a semi-chemical pulping system 100 thatincludes a digester 110. The inputs to the digester 110 include aconduit 102, which generally contains at least green liquor and uncookedpulp comprising lignocellulosic material. Conduit 102 (or anotherconduit) may supply other chemicals, such as, for example, NaOH, Na₂CO₃,Na₂SO₃, and/or white liquor, to the digester 110. The inputs to thedigester 110 also include conduit 108, which supplies white liquor, NaOH(and/or other bases), and/or other agents that may increase thesolubility of lignin and/or decrease the likelihood of extraction screenclogging.

The pulp is cooked in digester 110, which contains various extractionscreens. Though digester 110 may contain various extraction screens atdifferent elevations, extraction screen 112 illustrates where the spentor used liquor (i.e., black liquor) is removed or extracted from thedigester 110. Black liquor is extracted via conduit 114, which splitsinto conduit 116 and conduit 106. The flow content of conduit 116 ismixed with the flow content of conduit 108 (e.g., white liquor and/orNaOH) and recycled back into digester 110 via conduit 118. This recycleloop may facilitate the efficient extraction of lignin in the blackliquor while perhaps reducing the amount of white liquor and/or NaOHrequired.

The end of conduit 118 is positioned at or near the extraction screen112. The end of conduit 118 may deliver the contents of conduit 118 in aconcentrated location at or near extraction screen 112 or may deliverthe contents at or near substantially the entire surface of extractionscreen 112 within the digester. Alternatively, the end of conduit 118may be incorporated into the extraction screen 112. The distance fromthe addition point to the extraction screen may be such that the pH isincreased across substantially the entire surface of the extractionscreen so as to inhibit the adherence of lignin to the extractionscreen.

FIG. 2 shows a portion of another semi-chemical pulping system 200 thatincludes a digester 210. The inputs to the digester 210 include conduit202, which generally contains at least green liquor and uncooked pulp.The conduit 202 may also supply other chemicals, such as, for example,NaOH, Na₂CO₃, Na₂SO₃, and/or white liquor. The inputs to the digester210 also include liquor such as white liquor, NaOH (and/or other bases),and/or other agents through the conduit 208 that may increase thesolubility of lignin and/or decrease the likelihood of extraction screenclogging.

The pulp is cooked in digester 210, which contains various extractionscreens. Though digester 210 may contain various extraction screens atdifferent elevations, the extraction screen 212 illustrates where thespent or used liquor (i.e., black liquor) is removed or extracted fromthe digester 210. In the illustrated embodiment, black liquor isextracted via conduit 206.

The liquor supply conduit 208 feeds into and/or through (as illustrated)a center pipe 220 of the digester. The end of liquor supply conduit 208and/or center pipe 220 is positioned at or near extraction screen 212.The end of liquor supply conduit 208 and/or center pipe 220 may deliverthe contents of the liquor supply conduit 208 and/or center pipe 220 ina concentrated location at or near extraction screen 212 or may deliverthe contents at or near the substantially the entire surface ofextraction screen 212. Alternatively, the end of liquor supply conduit208 and/or center pipe 220 may be incorporated into extraction screen212. The distance from the addition point to the extraction screen maybe such that the pH is increased across substantially the entire surfaceof the extraction screen so as to inhibit the adherence of lignin to theextraction screen.

In certain embodiments, furthermore, it may be possible to combine therecycle loop and center pipe in a single method or vessel.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for inhibiting fouling of a black liquor extraction screenin a cooking vessel for semi-chemical pulping, the method comprising thesteps of: feeding lignocellulosic material and green liquor to thecooking vessel for semi-chemical pulping; cooking the lignocellulosicmaterial in the cooking vessel for semi-chemical pulping such that someor all lignin is separated from the lignocellulosic material, andwherein lignin is dissolved into a cooking liquor; extracting via anextraction screen black liquor comprising dissolved lignin, wherein theblack liquor is formed during the semi-chemical pulping process from thecooking vessel; and adding white liquor or a base to the cooking vesselat or near the extraction screen, such that a pH at or near theextraction screen is elevated and inhibits lignin from adhering to theextraction screen.
 2. The method of claim 1, wherein the step of addingwhite liquor or a base comprises adding white liquor and a base to thecooking vessel.
 3. The method of claim 1, wherein the base comprisesNaOH.
 4. The method of claim 1, wherein the step of feedinglignocellulosic material to the cooking vessel further comprises feedingat least one of NaOH, Na₂CO₃, Na₂SO₃ or white liquor to the cookingvessel.
 5. The method of claim 1, wherein the step of adding whiteliquor or the base to the cooking vessel at or near the extractionscreen occurs near the end of a cook when most lignin has been extractedfrom the lignocellulosic material.
 6. The method of claim 1, wherein thestep of adding white liquor or the base to the cooking vessel at or nearthe extraction screen occurs using a central pipe running through thecenter of the cooking vessel.
 7. The method of claim 1, wherein the stepof adding white liquor or the base to the cooking vessel at or near theextraction screen occurs using a circulation loop comprising a mixtureof white liquor or NaOH and black liquor comprising dissolved lignin. 8.The method of claim 1, wherein the step of adding white liquor or thebase to the cooking vessel at or near the extraction screen creates alocalized elevated pH at or near the extraction screen higher than thepH of the cooking liquor above the extraction screen.
 9. The method ofclaim 1, wherein a pH of a cooking liquor at an end of a cook is 10 orhigher
 10. The method of claim 1, wherein a pH of a cooking liquor at anend of a cook is between 10 and
 13. 11. The method of claim 1, wherein apH of a cooking liquor at an end of a cook is between 10 and
 12. 12. Acooking vessel for a process of semi-chemical pulping of lignocellulosicmaterial that inhibits fouling of a black liquor extraction screen inthe cooking vessel, the cooking vessel comprising: a first input conduitto the cooking vessel that carries green liquor; a second input conduitto the cooking vessel that carries lignocellulosic material; a cavity inwhich lignin is separated from the lignocellulosic material anddissolved into a cooking liquor; an extraction screen connected to anextraction conduit that extracts black liquor comprising dissolvedlignin, wherein the black liquor is formed during the semi-chemicalpulping process from the cooking vessel; and a third input conduit fordelivering white liquor or a base to the cooking vessel at or near theextraction screen, such that a pH at or near the extraction screen iselevated and inhibits lignin from adhering to the extraction screen. 13.The cooking vessel of claim 12, wherein the third input conduit deliverswhite liquor and a base.
 14. The cooking vessel of claim 12, wherein thebase comprises NaOH.
 15. The cooking vessel of claim 12, wherein thefirst and second input conduits comprise a single input conduit.
 16. Thecooking vessel of claim 12 further comprising a fourth input conduit forfeeding at least one of NaOH, Na₂CO₃, Na₂SO₃ or white liquor to thecooking.
 17. The cooking vessel of claim 12, wherein the extractionscreen is located near the end of a cook where most lignin has beenextracted from the lignocellulosic material.
 18. The cooking vessel ofclaim 12, wherein the third input conduit comprises a central piperunning through the center of the cooking vessel.
 19. The cooking vesselof claim 12, wherein the third input conduit comprises a circulationloop for recirculating a portion of the black liquor with a mixture ofwhite liquor or the base.
 20. The cooking vessel of claim 12, whereinthe third input conduit is located at or near the extraction screen soas to create a localized elevated pH at or near the extraction screenhigher than the pH of the cooking liquor above the extraction screen.